Catalytic cracking process for the treatment of a fraction having a low conradson carbon residue

ABSTRACT

Process for the fluidized-bed catalytic cracking of a weakly coking feedstock having a Conradson carbon residue equal to or less than 0.1% by weight and a hydrogen content equal to or greater than 12.7% by weight, comprising at least a step of cracking the feedstock, a step of separating/stripping the effluents from the coked catalyst particles and a step of regenerating said particles, the process being characterized in that at least one coking, carbonaceous and/or hydrocarbonaceous effluent having a content of aromatic compounds of greater than 50% by weight, comprising more than 20% by weight of polyaromatic compounds, is recycled to homogeneously distributed and weakly coked catalyst, before regeneration, in order to adjust the delta coke of the process.

FIELD OF THE INVENTION

The field of the present invention is that of the catalytic cracking ofpetroleum fractions, more particularly fractions which have a lowConradson carbon residue and a high hydrogen content and which,consequently, make it difficult to obtain the heat balance of the unit.

In a FCC (fluid catalytic cracking) unit, the heat balance is providedby the combustion of coke deposited on the catalyst during the reactionstep. This combustion takes place in the regeneration zone. Typically,the catalyst enters the regeneration zone with a coke content (definedas the ratio of the mass of coke to the mass of catalyst expressed as apercentage by weight) of between 0.5 and 1% by weight and leaves saidzone with a coke content of between 0.1 and 0.5% by weight forregenerators operating in partial combustion mode or between 0.1 and0.05% by weight, or even less than 0.01% by weight, for regeneratorsoperating in complete combustion mode.

In complete-combustion regeneration, all of the coke is burnt (typicalCO (carbon monoxide) content in the flue gas close to zero) whereas inpartial combustion mode the combustion of the coke produces CO with acontent of a few percent by volume, typically 0.5 to 10% by volume,depending on the air throughput and the degree of completeness of thecombustion in the case of incomplete combustion.

The Conradson carbon residue (or CCR) of the feedstock is defined by theASTM D 482 standard and represents, for those skilled in the art, ameasure of the amount of coke that the feedstock may produce during thecatalytic cracking reaction that takes place in the main reactor of theunit. Depending on the Conradson carbon residue of the feedstock, it ispossible to size the unit for a coke yield corresponding to the crackingof the feedstock so as to meet the heat balance of the unit that willcontrol the correct operation thereof.

Conventional heavy fractions treated in an FCC unit generally haveConradson carbon residues lying in the range from 0.2 to 10% by weight.

The fractions treated in an FCC unit according to the present inventionmay have a Conradson carbon residue equal to or less than 0.1% by weightand a hydrogen content equal to or greater than 12.7% by weight.

EXAMINATION OF THE PRIOR ART

To equilibrate the heat balance, those skilled in the art know to pushthe combustion in the regenerator by injecting thereinto more air for agiven amount of coke, i.e. to reduce the volume percentage of carbonmonoxide (CO) in the flue gas, which contributes to increase thetemperature of the catalyst within said regenerator and necessarilyhelps to meet the heat balance of the unit. However, it is not necessaryto increase the injected air quantity more than the quantity necessaryto the combustion of the coke present on the coked catalyst of the densebed in the regenerator. The combustion of coke allows an increase of thetemperature of the regenerated catalyst up to the desired crackingtemperature of the feed, and thus an equilibrium of the heat balance ofthe unit.

When this increase of the injected air volume is not sufficient orpossible, it is known in the prior art to recycle into the regenerator afraction resulting from the FCC having a high coke potential, known as acoking fraction, said fraction being introduced directly within theregenerator. This coking fraction is generally a fraction resulting fromthe cracking of the feedstock which is generally the “slurry” fraction,i.e. a predominantly aromatic 360° C.+ fraction, or any hydrocarbonfraction such as fuel oil No. 2 or domestic fuel oil. This recycling ofa coking fraction to the regenerator, common practice in the start-upphases of the unit, is tricky and is a source of problems duringcontinuous use. This is because, since the temperatures in theregenerator are around 650° C. to 750° C., a portion of the recyclevaporizes, forming cracked gases that will be found in the dilute phaseof the regenerator where they thus run the risk of creating hot spotsthat may be damaging to the proper operation of the regenerator. Thisphenomenon, often called “afterburning” or “post-combustion”, may bedefined as further combustion at an undesired point in the regenerator,for example in the dilute phase where the solid catalyst is present in asmaller amount, or at the inlet of or inside one of the cyclones alsopresent in the enclosure of the regenerator, or else in the combustiongas discharge lines. This term “afterburning”, well accepted andpracticed by those skilled in the art, will be used in the rest of thetext.

Moreover, this recycle stream runs the risk of burning in the catalystbed, locally forming a high-temperature flame front. This flame frontgenerates hot spots with locally high temperatures within the catalystbed. Since steam also forms when these hydrocarbons are burnt, theselocal high temperatures combined with the presence of steam weaken theactive part of the catalyst (zeolite) and thus deactivate its crackingfunction. It is referred to as hydrothermal catalyst deactivation. It isobserved that the richer this recycled fraction is in hydrogen (thelighter the fraction is, the higher its hydrogen content is, generatingmore steam by combustion), the greater the generation of steam is thatresults from its combustion.

It is also known to recycle hydrocarbons of tar or coke type to thestripper either in order to optimize the production of petrol andolefins via the use of bifunctional catalysts with recycle ofhydrocarbons (U.S. Pat. No. 3,856,659) or else to use the exothermicityof the recycle in order to improve the stripping of the coked catalystparticles in the stripper (U.S. Pat. No. 4,888,103). The optimization ofthe coking reaction on the initially coked catalyst by the feedstock,with a view to treating weakly coking feedstocks which do not make itpossible to obtain the thermal equilibrium necessary for the properoperation of the catalytic cracking process, is not mentioned in any ofthese documents.

In patent EP 2072605, the regenerator is used as a synthesis gasgenerator: for this purpose, a quantity of coking fraction on thecatalyst coked by the feedstock is recycled to the stripper. However,this quantity is much greater than that needed for the normalfunctioning of a regenerator in combustion making it possible tomaintain the temperature of the exiting regenerated catalyst between 690and 750° C., which temperature makes it possible to ensure the thermalequilibrium of the catalytic cracking unit. In order to consume theexcess coke formed by the recycle and in order to limit CO₂ production,not only is oxygen from the air injected into the regenerator but alsosteam in order to form the synthesis gas by consuming the coke. Sincethe gasification reaction is highly endothermic, there is no increase ofthe temperature beyond the critical threshold. In this document, the FCCprocess is considered to be a means of eliminating the CO₂ released atthe regenerator.

The present invention therefore targets the catalytic cracking of aweakly coking fraction comprising the recycling of at least one cokingfraction that makes it possible to increase the amount of coke in thecatalyst before its entry into the regenerator operating in combustionmode, but that also makes it possible to avoid the formation of hotspots in the dilute phase of the fluidized bed (afterburning) and todeactivate the catalyst (hot spots in the dense phase within theregenerator or stripper), the recycle taking place in a zone for thehomogeneous distribution of the coked catalyst particles.

Another objective of the present invention is to prevent the phenomenonof “afterburning” and of steam described previously which takes place inthe regenerator by limiting the amount of hydrogen-rich lighthydrocarbons that might have been entrained in the coke.

BRIEF DESCRIPTION OF THE INVENTION

The present invention applies both to FCC units using a reactoroperating in upflow mode (called a “riser” reactor) and to units using areactor operating in downflow mode (called a “downer” reactor), butalways to units in which the regenerator operates in the combustionmode.

The present invention also applies to FCC units operating with a singlereactor (in upflow mode or downflow mode) and to FCC units operatingwith two or more reactors. In general, when the FCC units operate withtwo reactors—a main reactor and a secondary reactor—if they operate inmaximum petrol or in maximum LCO mode, these reactors are riserreactors, but a unit operating with two downer reactors or with oneriser reactor and one downer reactor would not be outside the scope ofthe present invention.

The feedstocks that an FCC unit according to the present invention cantreat are feedstocks having a Conradson carbon residue equal to or lessthan 0.1% by weight and a hydrogen content equal to or greater than12.7% by weight.

The present invention can be described as a process for the catalyticcracking of a weakly coking feedstock having a Conradson carbon residueequal to or less than 0.1% by weight and a hydrogen content equal to orgreater than 12.7% by weight, comprising at least a feedstock crackingstep, a step for separating/stripping the effluents from the cokedcatalyst particles, a step for regenerating said particles by partial orcomplete combustion of the coke, and the recycle of at least a coking,hydrocarbonaceous effluent to catalyst that is homogeneously distributedand weakly coked by said feedstock, before regeneration, thecharacteristic of the process being that the amount of coking effluentinjected into the coked catalyst is adjusted so as to deliver anadditional amount of coke Q_(r) to the catalyst so as to satisfy thefollowing equation (I):

Q _(t) =Q _(i) +Q _(r)  (I),

in which Q_(i) is the initial coke content of the coked catalyst afterthe feedstock has been cracked and Q_(t) or delta coke is the cokecontent burned by partial or complete combustion, necessary formaintaining the heat balance of the process and for maintaining thetemperature of the regenerated catalyst at a temperature equal to ormore than 690° C., preferably varying from 690° C. to 750° C., saidcoking effluent having a content of aromatic compounds equal to orgreater than 50% by weight, comprising 20% by weight or more orpolyaromatic compounds, also referred to subsequently in the presentdescription as “coking fraction”.

The expression “polyaromatic compound” is understood to mean a compoundcomprising at least two aromatic rings with two common vicinal carbonatoms. A weakly coked catalyst is a catalyst for which the amount ofcoke obtained by cracking a feedstock is not large enough to maintainthe heat balance of the catalytic cracking unit in which it is used.Specifically, the regeneration of the catalyst, by burning off the coke,releases heat that should be recovered in sufficient amount by thecatalyst so that the latter supplies, on the one hand, energy sufficientto vaporize almost completely the feedstock injected in liquid form intothe reactor and supplies, on the other hand, sufficient energy to thegenerally endothermic cracking reactions so as to maintain a reactiontemperature at the outlet of said reactor which is generally between 480and 650° C. depending on the desired conversion objectives andconfigurations.

The advantage of the present invention is essentially that the amount ofcoke that is deposited homogeneously on the catalyst particles beforethey enter the regenerator of the unit is increased. This increase inthe coke (or delta coke) to be burnt off in the regenerator has theeffect of increasing the heat resulting from the combustion of the cokeand consequently of homogeneously increasing the temperature of theresulting regenerated catalyst particles that will be recycled into themain reactor without creating hot spots that are damaging to thecatalytic activity. The final advantage is that when the feedstocksintroduced into the main reactor do not form enough coke for thecracking, the recycle of coking hydrocarbonaceous effluent makes itpossible for the amount of coke or delta coke needed for thermalequilibrium of the unit to be adjusted, that is the recycle of cokinghydrocarbonaceous effluent makes it possible for the temperature of theregenerated catalyst leaving the regenerator to be adjusted, and thus toensure that said unit operates efficiently, even when weakly cokingfeedstocks are cracked.

For efficient operation of the FCC unit fed with a weakly cokingfeedstock, the amount of coke (Q_(t)) present on the catalyst enteringthe regenerator, necessary for equilibrating the heat balance, will haveto correspond to the sum of the initial amount of coke (Q_(i)) suppliedby the cracking of the feedstock over the catalyst (in the mainreactor(s)) and of the additional amount of coke (Q_(r)) supplied byrecycling the effluent to the coked catalyst after the feedstock hasbeen cracked.

Generally, Q_(t), the amount of coke trapped on the catalyst, or “deltacoke”, entering into the regenerator, typical for a balanced heatbalance may be allowed to vary between 0.5 and 1.4% by weight (limitsincluded). For attaining thermal equilibrium, that is a temperature ofthe regenerated catalyst which will be in contact with the feedstock, isequal to or more than 690° C. (varying for example from 690° C. to 750°C.), the injected air quantity will have to be adjusted according to theamount of coked which has been formed.

Preferably, Q_(t) is kept between 0.5 and 1.1% by weight (limitsincluded) when the combustion takes place in a single-stage regeneratorwith complete combustion, and between 0.8 and 1.4% by weight (limitsincluded) for a partial combustion in a first stage of a multistageregenerator comprising at least two regeneration stages.

To implement the invention, the hydrocarbonaceous effluent having anaromatic content equal to or greater than 50% by weight, comprising 20%by weight or more or polyaromatic compounds, also called coking fractionsubsequently in the present description, is a predominantly aromatic,carbonaceous and/or hydrocarbonaceous effluent, the boiling point ofwhich is equal to or greater than 220° C., and preferably equal to orgreater than 340° C., such as LCO (light cycle oil) or HCO (heavy cycleoil) fractions with a distillation range typically between 360 and 440°C., and “slurry” (residual sludge from catalytic cracking) with adistillation range equal to or greater than 360° C. or 360° C.+, thefractions of finished products of heavy fuel oil type, such as Fuel oilNo. 2, intermediate fractions resulting from atmospheric distillation orvacuum distillation such as distillation residues, or else highlyaromatic fractions resulting from the conversion of crude oil, biomassresulting from the conversion of wood and/or cellulose, petroleum cokeor biomass that is powdered, dispersed in or sprayed into a fluid bydilution or blowing, asphalt-rich fractions coming from deasphaltingunits, waxes resulting from the liquefaction of coal by an indirect(GTL) process or from a Fischer-Tropsch process for converting gas intohydrocarbons, petroleum coke, or a mixture of said fractions.

Among the weakly coking feedstocks that the present invention can treatmay be found the following:

-   -   purges from a hydrocracker unit, called bleeds, having a        hydrogen content equal to or greater than 12.7% by weight as        they are highly paraffinic;    -   severely pretreated VGO (vacuum gas oil) feedstocks (resulting        from the vacuum distillation of atmospheric distillation        residues), having a boiling point equal to or greater than        350° C. and having hydrogen contents equal to or greater than        12.7% by weight;    -   vegetable oils; and    -   paraffins resulting from the Fischer-Tropsch process;

it being possible for these feedstocks to be cracked individually or asa mixture in the main and/or secondary reactor of the catalytic crackingunit.

The present invention involves the production of effluents such as, forexample, petrol from a feedstock having a Conradson carbon residue equalto or less than 0.1% by weight and a hydrogen content equal to orgreater than 12.7% by weight, by fluid catalytic cracking (FCC), thecorresponding unit having at least one main reactor operating in upflowmode (riser reactor) or in downflow mode (downer reactor), the cokedcatalyst leaving the reactor being introduced into aseparating/stripping zone in which the coked catalyst is separated fromthe cracking effluents, then recovered in the stripping step, in thestripper of the unit. Said stripping step operating in a fluidized bedand having a dense phase surmounted by a dilute phase, the recycle ofhydrocarbonaceous effluents, or coking fraction, is carried out in atleast one zone, referred to as a recycle zone, by means of at least onedispersion device within the dense phase of the stripper. However, inorder for this diffusion to be optimal and for there to be no hot spotswith risks of over-coking which may give rise to hot spots later duringthe combustion in the regenerator, the dense catalyst phasecorresponding to the coking effluent recycle zone is homogenized byinserting at least one structured packing element that improves thedispersion of the coked catalyst particles located upstream of thedispersing of the recycle relative to the stream of catalyst particlesand that prevents the departure of coke particles being formed at thesame time as the cracking effluents. These structured packing elementsmay cover all or part of the cross section of said stripping zone andover at least a portion of the height thereof, possibly in a stagedmanner, at least upstream and optionally downstream of the injection ofat least one coking fraction.

According to a first variant of the present invention, the strippingstep will comprise at least two zones comprising at least a firstrecycle coking zone occupied by at least a first structured packingelement located upstream of the dispersing of the recycle of cokingfraction and at least a second stripping zone occupied by at least asecond structured packing element located downstream of the inlet ofsaid recycle but upstream of the dispersing of the stripping fluidsneeded for discharging the cracking effluents. In each of the zones, itis possible to have a superposition of packing elements as a function ofthe desired homogeneity of the catalyst. It would not be outside thescope of the present invention if the coking fraction was recycled to atleast two zones, these two zones being equipped with two superpositionsof different structured packing elements, each of these zones comprisingdevices for dispersing the coking fraction downstream of the stream ofcatalyst particles, these zones always being followed by a thirdstripping zone corresponding to a third superposition of packingelements associated with a device for dispersing the stripping fluid.

In the context of the invention, according to a second variant, thecoking fraction recycle zones may be separated by stripping zones, forexample one or more recycle zone(s) then a stripping zone. In thissequence of recycle and stripping zones, the last zone is always astripping zone.

By using structured packing elements it is possible to provide acontinuous catalyst stream of homogeneous density. In a preferredembodiment, these packings occupy less than 10% of the area of the flowcross section in the vessel in which they are placed, although inprojection on said section they occupy the entire area thereof.

The expression “stripping fluid” is understood to mean any compound nothydrocarbonaceous that is in the gas state at the time it is injectedinto the stripper, chosen from inert gases and steam. They ensure theaeration of catalyst particles but also make it possible to eliminatehydrocarbons trapped in the bed and/or the particles, which has theeffect of increasing the catalytic activity of these particles.

In one preferred embodiment of the invention, a pre-stripping zoneoccupied by at least one structured packing element associated with adevice for dispersing at least one stripping fluid downstream of thestream of particles is positioned upstream of the first recycle zone.The addition of this pre-stripping zone thus helps to restore aconsiderable portion of its catalytic activity to the catalyst andtherefore of its coking capacity in the stripper.

Another subject of the present invention is a plant for implementing theinvention, comprising the various vessels needed to implement acatalytic cracking process, that is to say at least a main reactor andpossibly at least a secondary reactor, at least a disengager and astripper and a single-stage or multistage regenerator, said plantincluding, in the stripper part, at the dense catalyst bed, at least onezone equipped with at least one structured packing element positionedupstream of the device for dispersing the coking fraction relative tothe circulation of the stream of catalyst particles and these packingelements being formed by interlacing plates, strips or fins constitutinga screen, this screen occupying less than 10% of the area of the flowcross section of the vessel in which it is placed but, in projection onsaid section, it may cover the entire area thereof.

Preferably, the stripper part of the plant will contain at least twozones equipped with structured packing elements associated with twofluid-dispersing devices, one for dispersing coking fractions, the otherfor dispersing the stripping fluid, these devices being locateddownstream of said packing elements relative to the stream of catalystparticles.

In one variant, several superpositions of packing elements, eachassociated with a device for dispersing a coking fraction, may come oneafter another with at least one packing element associated with a devicefor dispersing the stripping fluid in one and the same stripping step.

As packing elements, one or more of the structured packing elementsdescribed in the patents EP 719 850, U.S. Pat. No. 7,022,221, U.S. Pat.No. 7,077,997, WO 2007/094771, WO 00/35575 and CN 1 763 150 may be used.Here, in each of the envisaged packings, the stream of coked particlesis aerated by making them follow preferential pathways obtained byinterlacing plates, strips or fins constituting a screen. This screenmay occupy less than 10% of the area of the flow cross section of thevessel in which it is placed but, in projection on said section, it maycover the entire area thereof. Such interlacing is generally arranged inlayers of the same type, enabling this aerating of the particles to becontrolled.

The devices for dispersing the recycle or stripping fluids may be chosenfrom spraying rod-type injectors, rings and sparger tubes.

For the injection of heavy fractions, it is preferred to use injectorsfor example of the venturi type, which, owing to a pressurizedco-injection of a dispersing fluid, generally steam, make it possible toatomize the recycle fluid in order to thereby accelerate itsvaporization immediately after it is injected.

In a first variant of said plant, the stripper may be located in thesame vessel as the disengager.

In a second variant, the stripper may be located in a different vesselto the disengager, located downstream of the latter, but stillpositioned upstream of the regenerator. In one preferred form of theplant, the disengager and/or the stripper will advantageously comprise,at the outlet and/or inlet of the catalyst particles, at least onestructured packing element followed by a device for dispersing thestripping fluids for the pre-stripping of the catalyst particles.Moreover, the stripping vessel will comprise means for discharging thecracking effluents essentially resulting from the recycle of the cokingfractions to the coked catalyst in said vessel.

When the stripper comprises a plurality of packings for the intercalatedrecycle and stripping, each comprising devices for dispersing recyclefluids and stripping fluids, the volumes occupied for the recycle andthe stripping are respectively from 25 to 65% and from 35 to 75% of thevolume of the zone or vessel corresponding to the stripping step.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described with reference to the appendednon-limiting drawings in which:

FIG. 1 is a section through a disengager-stripper in a single vessel;

FIG. 2 is a diagram representing a catalytic cracking unit with twocracking reactors, a primary reactor and a secondary reactor, for whichthe stripping vessel is independent of the vessel for disengaging thecoked catalyst/cracking effluents mixture.

FIG. 3 represents, in cross section, two methods of filling strippingvessels with structured packings, each associated with a dispersiondevice downstream.

FIG. 1 represents one embodiment of the reactor/stripper part of theplant for implementing the invention: it comprises a riser reactor (R1)equipped at its upper end with the single disengager/stripper (1)comprising the disengager part (1 a) and the stripper part (1 b) in thesame vessel. This stripper part is equipped with three packings composedof several structured elements (I₁, I₂ and I₃). The stream of catalystparticles circulating from top to bottom, downstream of each packinghave either steam dispersion rings (D₁ and D₂) for stripping thecatalyst particles or a hydrocarbonaceous compound recycle injector (2).The pipe (8) connects the disengager/stripper (1) to a regenerator (notshown).

FIG. 2 represents the whole of an FCC unit implementing the process ofthe invention according to one particular mode. The unit as representedcomprises two reactors (R₁, main reactor, and R₂) both fed with afeedstock (C₁, C₂, C₁ being a feedstock according to the presentinvention). The effluents and the catalysts coked by the feedstocks inthe two riser reactors are brought together in one and the samedisengager (1). The unit also comprises a separate and independentstripper (5), connected to the disengager (1) via the pipe (7), and tothe two-stage regenerator (3 and 4) via the pipe (8). The stripper (5)is equipped with three packings composed of several structured elements(I₁, I₂ and I₃). Within the stripper (5), with the stream of catalystparticles circulating from top to bottom, downstream of each packingeither steam dispersion rings for stripping the catalyst particles (D₁and D₂) or a hydrocarbonaceous compound recycle injector (2) areintroduced.

FIG. 3 represents, according to sections A-A and B-B, two methods offilling the stripper with packings composed of a non-identical number ofstructured elements. In these two sections, the stream of catalystparticles circulates from top to bottom. According to the section A-A,two successive packings (I₂) and (I₃) with injection of a recycle ofhydrocarbons (10) and (11) via an injector, and a packing I₁ forstripping the particles by steam dispersion via the dispersion ring D₁.According to the section B-B, the stripper is equipped with threepackings composed of several structured elements, two stripping stepscorresponding to packings (I′₂) and (I′₃) and to the dispersion rings(D′₁) and (D′₂) frame a coking step comprising the recycle ofhydrocarbon via the injector (10) and the packing (I′₂).

The examples, like the figures described above, aim to describe theinvention without limiting the scope thereof.

EXAMPLE 1

The present example shows the advantages of the present invention bycomparing the efficiency in terms of product yield when weakly cokingfeedstocks are cracked in an FCC unit with or without recycle of cokingfractions.

A base case may be distinguished in which there is no recycle with afluid catalytic cracking (FCC) unit having a single riser reactor with acapacity of 40 000 barrels per day, i.e. 240 tonnes per hour, andtreating a corresponding hydrotreated VGO feedstock.

The main properties of the feedstock are given in Table 1 below.

TABLE 1 Feedstock Hydrotreated VGO Density g/cm³ 0.8610 H₂ content wt %13.5 Sulphur content ppm by weight 330 Nitrogen content ppm by weight550 CCR (Conradson <0.1 carbon residue) Ni content ppm by weight <2 Vcontent ppm by weight <2

This unit with no recycle of “coking” fraction into the stripper iscarried out under the conditions presented in Table 2.

TABLE 2 C/O 8.6 Riser outlet temperature, ° C. 525 delta coke wt % 0.60Regenerator temperature, ° C. 671

The regeneration temperature is too low, which may cause afterburning orpost-combustion reactions of the coke which is only incompletely burntoff. Indeed, additional combustions may take place in the dilute phaseof the bed fluidized in the regenerator, by combustion of the entrainedparticulate coke following the incomplete combustion in the dense phasethereof. In order to obtain a complete combustion, the optimalregeneration temperature required to prevent such phenomena is generallyequal to or greater than 690° C.

The associated yield structure, that is to say the amounts of productsobtained by cracking the feedstock, is provided in Table 3.

TABLE 3 Yield relative to the feedstock wt % Dry gases 1.98 LPG C3/C422.81 Petrol C5-220° C. 56.50 LCO (220-360° C.) 9.53 >360 ° C. 3.98 Coke5.22

In the second case, a “slurry” fraction resulting from the unit itselfis recycled, as described in FIG. 1, to the dense phase of the stripper(1 b), via 4 dispersion devices (2) positioned equidistantly downstreamof a first packing (I₂) comprising several structured elements that makeit possible to homogenize the stream of descending catalyst particlesand to obtain good contact between the latter and the recycled slurry,and thus a thoroughly homogeneous deposit of additional coke on thecatalyst. The positioning of the dispersion devices in the stripperdownstream of the packing with which they are associated is chosen sothat the overall contact time between the slurry and the catalystparticles is 70 seconds for a descent rate of the catalyst particles of65 kg/m²/s.

A second packing (I₃) is located in the lower part of the dense phase ofthe stripper (1 a) associated with a device for dispersing a strippingfluid (D₁), here, steam: the dispersion of steam makes it possible tostrip the light products loaded with hydrogen atoms resulting from thecracking of the coking fraction. These light hydrocarbons will berecovered and mixed with the effluents from the reactor (R₁) in order tothen be distilled and finally upgraded in the refinery. In this way, thecoke (Q_(r)) resulting from the coking of the polycondensed orpolyaromatic heavy hydrocarbons that are not very rich in hydrogen, isadded to the coke (Q_(i)) resulting from the cracking of the feedstockin the reactor (R₁) in order to constitute the amount of coke (Qt)needed for the heat balance of the unit, before being sent to theregenerator. As this additional coke is free of an excess of hydrogendue to the stripping after the cracking reaction, the risks of hot spotsappearing that are damaging to the catalyst, linked to the combustion ofthe hydrogen and also an excessive production of steam in theregenerator, will be avoided.

A third packing (I₁) associated with a device for dispersing strippingfluid (D₂), mainly steam, is positioned upstream of the first packing(I₂) in the dense phase of the stripper (1 a) in order to carry out apre-stripping of the catalyst particles before they encounter saidcoking fraction and thus help to restore a considerable portion of thecatalytic activity and therefore of the coking power of said catalystparticles. The positioning of the devices for dispersing steamcorresponds to that of the devices for dispersing the coking fraction:the targeted overall contact time between the stripping fluid and thecatalyst particles is 70 seconds for a descent rate of the catalystparticles of 65 kg/m²/s.

Collated in Table 4 below are the yields obtained for the recycling of aslurry to a dense phase of catalyst particles in the stripper when thereis:

-   -   State 1; neither prior stripping (or pre-stripping I₃+D₂), nor        packing upstream of the recycle, but a terminal stripping        (I₁+D₁)    -   State 2; no pre-stripping (I₃+D₂), a packing (I₂) upstream of        the dispersion device (2) for the recycle of slurry and finally        a terminal stripping (I₁+D₁).    -   State 3; a pre-stripping (I₃+D₂), followed by the recycle of        slurry (I₂+2) and finally a terminal stripping (I₁+D₁).

TABLE 4 Yield relative to the feedstock State 1 State 2 State 3 Drygases (wt %) 1.94 2.32 2.44 LPG C3/C4 (wt %) 2.44 3.18 3.66 PetrolC5-220 ° C. (wt %) 11.87 13.89 14.93 LCO (220-360° C.) (wt %) 29.0028.62 28.09 Slurry >360° C. (wt %) 39.54 33.13 31.40 Coke (wt %) 15.0918.86 19.81

In this table it is observed that the introduction of a packingcomprising structured elements upstream of the recycle of slurry makesit possible to increase the amount of coke that will be deposited on thecatalyst, and that the addition of a step of pre-stripping the catalystbefore bringing it into contact with the coking fraction makes itpossible to still further increase the cracking and the coking effect ofthis fraction.

In order to illustrate the contribution of the invention, Table 5 shows,for the unit in question, the gains as regards the amount of cokedeposited on the catalyst (Q_(t)), or else delta coke, and also thecorresponding increase in the temperature of the dense phase in theregenerator for a throughput of the coking fraction recycled to thestripper of 6 t/h.

TABLE 5 State 1 State 2 State 3 Regenerator temperature (° C.) 680 686691 Delta coke (wt %) 0.63 0.66 0.68 C/O (weight/weight) 8.2 7.9 7.7

Thus, depending on the configuration envisaged for the recycle of thecoking fraction to the stripper and therefore on the resulting amount ofcoke (see Table 4) deposited on the catalyst, the temperature within theregenerator increases from 671° C. for the configuration with no recycleto 691° C. for the “State 3” configuration and thus limits theafterburning phenomena linked to temperatures of the dense phase thatare too low, typically below 690° C.

EXAMPLE 2

The present example shows the advantage of the present invention formaking it possible to equilibrate the heat balance of a catalyticcracking unit with a deficit of coke in the regenerator operating incombustion mode by cracking of a weakly coking feedstock.

In this example, the catalytic cracking unit has a capacity of 340 t/hand treats a highly paraffinic feedstock originating from ahydrocracker. This feedstock has a density of 0.86, a Conradson carbonresidue, determined by the ASTM D 482 standard, of less than 0.1% byweight and a content of metals (nickel+vanadium) of less than 0.1 ppm.

In Table 6 below, the first column collates the characteristics of thisunit treating said feedstock with no recycle of heavy hydrocarbons intothe stripper. By calculating the heat balance of the unit, a very smallamount of coke on the catalyst, of 0.4% by weight, is obtained, whichresults in a very low temperature for the dense phase of the fluidizedbed in the regenerator, barely above 640° C. Increasing the injection ofair beyond the amount mentioned does not make it possible to increasethis temperature beyond this threshold.

To raise the temperature of the catalyst, a heavy hydrocarbon, in thiscase slurry (350+), the density of which is 1.083 and the Conradsoncarbon residue of which is greater than 10% by weight, originating fromthe bottom of the primary fractionating column of the catalytic crackingunit is recycled. This recycling consists in injecting said heavyhydrocarbon into the stripper at the inserts dividing the grains ofcatalyst coked by the feedstock. The results of the heat balance aregiven in the second column of Table 6.

It is observed that by recycling 20 t/h of slurry to the stripper, theamount of coke deposited on the catalyst via cracking increasessignificantly, which then makes it possible to obtain a dense phasetemperature which is perfectly satisfactory for ensuring the combustionof the coke on the catalyst via injection of a reasonable amount of air.

It is observed in this case that the hydrogen content of the cokeincreases slightly due to the adsorption of heavy molecules on thecatalyst in the stripper, the H/C (hydrogen/carbon) molecular ratio ofwhich is greater than that of the coke initially deposited on thecatalyst following the cracking of the feedstock in the reactor. Thisincrease of hydrogen in the coke is desired here because the combustionof this additional hydrogen helps to increase the temperature of thedense phase of the fluidized bed in the regenerator.

TABLE 6 Without recycle With Recycle Feedstock throughput t/h 340 340Heavy HC recycle t/h 0 20 throughput Preheat Temperature ° C. 250 250Reaction Temperature ° C. 518 518 C/O wt % 9.5 6.8 Delta coke (Qt) —0.51 0.72 H in coke wt % 6.94 7.35 Dense Phase ° C. 642 694 TemperatureInjected air throughput t/h 229 247

From Table 6 it is observed that the amount of coke increases on thecatalyst (Qt varying from 0.51 to 0.72) and that the excessively lowtemperature of the unit with no recycle is increased to more than 690°C., this ensuring the re-equilibration of the heat balance of the unit.

1.-12. (canceled)
 13. Plant for implementing a process for fluidized bedcatalytic cracking, comprising at least a main reactor and optionally atleast a secondary reactor, at least a disengager and a stripper, and asingle-stage or multistage regenerator, characterized in that thestripper contains, level with a dense catalyst bed, at least one zoneequipped with at least one structured packing element positionedupstream of a device for dispersing a coking fraction with respect tocirculation of a stream of catalyst particles, wherein said structuredpacking element(s) are formed by interlacing plates, strips or finsconstituting a screen, said screen occupying less than 10% of the areaof flow cross section in a vessel in which it is placed, but covering,in projection on said section, the entire area thereof.
 14. Plantaccording to claim 13, characterized in that the stripper contains atleast two zones equipped with at least one structured packing elementthat are associated with two fluid-dispersing devices, one fordispersing coking fractions, the other for dispersing stripping fluid,these devices being located downstream of said structured packingelements relative to the stream of catalyst particles.
 15. Plantaccording to claim 13, characterized in that the recycle and strippingdispersion devices are chosen from spraying, rod-type injectors, venturipressurized atomizing injectors, fluidization rings and sparger tubes.16. Plant according to claim 13, characterized in that the stripper islocated in one and the same vessel as the disengager.
 17. Plantaccording to claim 13, characterized in that the stripper is located ina different vessel downstream of the disengager, but still positionedupstream of the regenerator.
 18. Plant according to claim 17,characterized in that the disengager and/or the stripper comprises,respectively, at the outlet of said disengager and/or inlet of thecatalyst particles into said stripper, at least one structured packingelement followed by a device for dispersing stripping fluids for thepre-stripping of the catalyst particles.
 19. Plant according to claim13, characterized in that when the stripper comprises a plurality ofstructured packing elements for the intercalated recycle and stripping,each structured packing element being associated with a fluid-dispersingdevice, the volumes occupied for the recycle and the stripping arerespectively from 25 to 65% and from 35 to 75% of the volume ofstripping zone or vessel.